Electric charge quantization is a long-standing question in particle physics. While Grand Unified Theories (GUTs) have typically been thought to preclude the possibility for particles that do not have integer multiple electron charge (millicharged particles hereafter), well-motivated dark-sector models [1]}, [2]} have been proposed to predict the existence of millicharged particles while preserving the possibility for unification. Such models can contain a rich internal structure, providing candidate particles for dark matter. Recent results from the EDGES experiment [3]} suggest that the observed 21-cm absorption profile can be explained if a fraction of dark matter is composed of millicharged particles [4]}.
